1. Field
The disclosed concept pertains generally to circuit interrupters and, more particularly, to circuit breakers. The disclosed concept also pertains to miniature circuit breakers.
2. Background Information
Circuit interrupters, such as circuit breakers, are generally old and well known in the art. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Industrial circuit breakers often use a circuit breaker frame, which houses a trip unit. See, for example, U.S. Pat. Nos. 5,910,760; and 6,144,271. The trip unit may be modular and may be replaced, in order to alter the electrical properties of the circuit breaker.
It is well known to employ trip units which utilize a microprocessor to detect various types of overcurrent trip conditions and provide various protection functions, such as, for example, a long delay trip, a short delay trip, an instantaneous trip, and/or a ground fault trip. The long delay trip function protects the load served by the protected electrical system from overloads and/or overcurrents. The short delay trip function can be used to coordinate tripping of downstream circuit breakers in a hierarchy of circuit breakers. The instantaneous trip function protects the electrical conductors to which the circuit breaker is connected from damaging overcurrent conditions, such as short circuits. As implied, the ground fault trip function protects the electrical system from faults to ground.
The earliest electronic trip unit circuit designs utilized discrete components such as transistors, resistors and capacitors.
More recently, designs, such as disclosed in U.S. Pat. Nos. 4,428,022; and 5,525,985, have included microprocessors, which provide improved performance and flexibility. These digital systems sample the current waveforms periodically to generate a digital representation of the current. The microprocessor uses the samples to execute algorithms, which implement one or more current protection curves.
When diagnosing field issues with an arc fault circuit interrupter (AFCI), engineers often rely heavily on hearsay reports of the circumstances surrounding each issue. These reports can come from users, electricians and sales staff. Although the people providing the information are certainly well-intentioned and their efforts are greatly appreciated, the quality of information that gets reported back from the field is often of poor or questionable value. In fact, assessing the quality of information provided from field reports is often as big a challenge as determining what the original problem may have been.
When the pattern of available information is confusing or unclear, then engineers are forced to make very broad guesses as to what the field issue may have been. Hence, diagnosing a field issue is difficult with little solid information to help diagnose the issue. In these cases, it is often required to send a circuit interrupter design engineer to a field location along with oscilloscopes and other diagnostic equipment in order to collect additional firsthand information about the issue. This can be time consuming, costly and even unproductive if the field issue is not repeatable.
There is a need for a “black box” in a miniature circuit breaker, in order to improve the quantity and quality of information available when diagnosing, for example, AFCI issues encountered in the field.
In known miniature circuit breakers, the information that the circuit breaker uses to make each trip decision is lost because there is no comprehensive storage mechanism. For example, a known AFCI microprocessor stores only a single byte of information (i.e., the “cause-of-trip”) in its internal data EEPROM per trip event. This is because of various restrictions.
The highest priority of an AFCI is to interrupt the protected circuit whenever an exceptional condition is suspected. The processor cannot delay circuit interruption in order to store information. Hence, the microprocessor stores a “cause-of-trip” in EEPROM only after a fault has been identified and a signal has been sent to trip open the circuit breaker operating mechanism. Also, there is a limited time after the AFCI interrupts the protected circuit for the processor to store information. This is because the AFCI uses power provided by the utility source, which is interrupted when the circuit breaker separable contacts open. For example, the time required to store information in EEPROM is relatively large (e.g., about 5 to 10 milliseconds (mS)) when compared to the power supply hold time, such that only a single byte of information can be reliably saved for each trip event.
Another problem associated with EEPROM is that the single AFCI microprocessor may stop executing code while information is being written to its EEPROM. As a consequence, the processor does not write to EEPROM any time it is looking for faults. Otherwise, if this were allowed, then the microprocessor would be “blind” to arc fault conditions each time that it stored data. Furthermore, restrictions on the number of write cycles of EEPROM (e.g., 300,000 maximum write cycles), mean that a limited amount of information can be stored in EEPROM.
A conventional branch feeder arc fault circuit breaker provides protection for parallel arcs and 30 mA ground faults. This generally does not employ a processor, and does not provide data logging, extraction of a status log or user communications. Also, no cause-of-trip information is available.
A known first generation combination circuit breaker provides protection for parallel arcs, series arcs and 30 mA ground faults. This employs a processor, provides a single trip record containing one byte of information (i.e., the most recent cause-of-trip) in data EEPROM for data logging, and provides for extraction of the cause-of-trip by connecting a third party EEPROM development tool directly to the circuit breaker printed circuit board, but does not provide user communications. The cause-of-trip information is not available to the user.
A known second generation combination circuit breaker provides improved protection for parallel arcs and series arcs, and optionally 30 mA ground faults. This employs a processor, provides several hundred trip records, each record containing one byte of information indicating a cause-of-trip for each trip event in data EEPROM for data logging, and provides for extraction of the cause-of-trip by an optional blinking LED, but only for the most recent trip event. A status log of the full trip history is available by connecting a proprietary tool directly to the circuit breaker printed circuit board, but is not available to the user.
There is room for improvement in circuit interrupters.
There is also room for improvement in circuit breakers, such as miniature circuit breakers.